1. Technical Field
The field of this invention relates to electromagnetic acoustic transducers usable with substantially cylindrical objects, and methods for determining resonant frequencies and identifying physical properties, particularly flaws, of the substantially cylindrical objects using electromagnetic acoustic transducers with coils recessed into parallel arrays of magnets arranged in a radial fashion transverse to the longitudinal axis of the cylindrical objects.
2. Description of the Related Art
Inspections according to and as required by government regulations and industry standards are commonly performed on commercially manufactured products including steel and aluminum pipes and tubing and also including elongated, substantially cylindrical bar stock, such as those used in pressurized applications, and also including elongated, substantially cylindrical bar stock, such as those used in high torque applications, e.g., fastenings. The subjects of these regulations and standards are the qualitative and quantitative standards controlling the properties of materials, such as strength, granularity and the presence and severity of flaws.
Clearly, the presence of certain defects can adversely affect the safety and structural integrity of the finished product. It is therefore preferred that the flawed portions of these metallic stocks and commercially manufactured products be economically identified and removed with the flawed portions of these materials recycled. In addition, the need to correct and improve processes and machinery by identifying flaw-producing causal relationships motivates actions that subject the removed portions to further inspections prior to recycling.
It is well known that in the fabrication and refinement of metallic articles, the foundry process itself, as well as errant machinery and other equipment used in the fabrication and refinement can produce one or more unacceptable flaws in the product articles. Present common practice in accordance with current quality control programs places an entire production run subject to rejection due to flaws in a small, but statistically significant, number of parts. There exists economic motivation to detect such flaws directly and thereby indirectly identify one or more problems with the manufacturing equipment or process. Once identified and corrected, manufacturers may then work to minimize the future generation of flawed metallic structures as well as the magnitude of the rejected stock under test. Varying degrees of economic benefit may be gained by the use of various test techniques in ascertaining the location of a flaw and determining the type of flaw.
Presently, there exist a variety of test techniques available and known to practitioners in the field that manufacturers may implement separately or in combination to determine flaws in metallic structures. Generally, one may dichotomize these as destructive and non-destructive test techniques. In destructive testing, a selected portion of the subject metallic item is destroyed; a portion that could very well be flawless while the remaining unselected portions of the metallic part may contain numerous unacceptable flaws. The extensibility of the results of destructive testing of a portion of the metallic item to the remainder of the item can vary greatly.
The principal advantage of non-destructive testing is that the metallic structure is examined throughout with only the flawed portion being isolated and removed, thereby leaving the unflawed portion for immediate usage. Typically, once a flawed portion in the metallic structure is determined, the testing procedure and/or device employs some means to ascertain the location of that flaw and the metallic structure is marked accordingly. With respect to non-destructive test techniques, there is a wide range of technology available including the use of eddy currents, magnetic flux leakage, x-ray, ultrasound, neutron detraction, and so forth. Where commercially practical non-destructive testing is implemented, one skilled in the art typically places one or more of devices embodying at least one of these non-destructive technologies within the production line so as to ascertain a flawed portion of a particular metallic structure during the normal production of a metallic structure.
Practitioners in the field are generally aware of the use of acoustic resonance techniques in the non-destructive testing of metallic structures. The use of acoustic resonance can offer significant advantages over other prior art types of non-destructive testing technologies. However, acoustic resonance techniques of the prior art have met with limited success due to the difficulty and expense in applying such especially with the use of contact type transducers.
Contact-type transducers, by their mechanical nature, affect the intrinsic resonance of the metallic structure since such transducers, by necessity, come into mechanical contact with the metallic structure. The affect the testing device has on the metallic part under test leads to complex signal and processing schemes to account for the variation in resonance such as taught by U.S. Pat. No. 5,062,296 to Magliori and U.S. Pat. No. 4,976,148 to Magliori, et al. These inventions employ a ceramic piezoelectric transducer, one that is typically a contact-type of transducer. A non-destructive testing device that does not require contact with the metallic part under test would be preferred so that the metallic structures under test resonate in isolation and thereby forgo the complexities and uncertainties associable with the contact-type signal processing.
It is well known to those skilled in the art that non-destructive testing can be performed by electromagnetic acoustic transducer (EMAT) devices disclosed in U.S. Pat. No. 6,109,108 to Ohtani, et al., U.S. Pat. No. 6,164,137 to Hancock, et al., U.S. Pat. Nos. 5,895,856, 6,119,522 and 6,170,336 all to Johnson, et al. (collectively referred to as Johnson) and in U.S. Pat. No. 5,808,202 to Passarelli. Ohtani discloses burst wave technique with an apparatus using a sheet coil. Hancock discloses a technique for tubes using dual circumferential acoustic waves produced by a transmitting EMAT and sensed by a pair of receiving EMATs. Johnson and Passarelli disclose EMATs typically in arranged one or more planes of magnets of alternating polarities arrayed in a radial fashion about a substantially annular orifice or pass-through hole and transverse to the longitudinal axis of the substantially cylindrical object under test. Electrical wire is then positioned about each magnet array. Passarelli and Johnson disclose EMATs that can provide flaw detection at various levels within a metallic structure under test. This detection in depth offers significant advantages over Hancock when moving from tubular to substantially solid objects under test.
Passarelli discloses each plane pair, together with transmitting and receiving coils and signal processing, act as the transmitting and receive circuit and provide the fields producing the resonance frequencies in the metallic structures under test. Observations gathered from field trials in both steel and aluminum mills indicate that there is an unacceptable decrease in the distance from the metallic part under test to the transducer coils. That is, the gap distance is decreased for the larger sample sizes of metallic structures of one inch or greater in diameter or maximal transverse dimension, limiting the degree to which one may locate the transmitting and receiving coils in closest practical proximity to one another, as is done for smaller diameter metallic structures such as around one-eighth to three-eights inch in diameter. On the other hand, if the gap distance is practically minimized, the electrical benefit is that the field coupling into the metallic structure is maximized and the cross-talk leaking into an adjacent transducer is minimized.
Under typical milling conditions, a metallic structure such as a pipe or tube moves at around three hundred feet per minute relative to the transducer. In the course of ordinary milling condition, the metallic structure moves laterally, or wobbles, and it is this wobble, in practice, that causes the metallic structure to mechanically contact the transducer. That is, as the gap distance decreases, the metallic structure under test is at great risk of coming into contact with the transducer causing damage to the transducer and posing a hazard to mill operations. Thus, a disadvantage of Passarelli and Johnson is that one cannot both accommodate the larger stock and ensure, under continuous mill conditions, the safe passage of the material structure in conjunction with the transducer, because the gap distance from the metallic part to the transducer coil cannot be practicably decreased within the prior art to acceptable distances.
There is another disadvantage to the aforementioned prior art arrangement in that the geometry of the coil used in conjunction with the transducer has an aspect ratio, thin and wide, that naturally causes the solenoid coil to have a substantial field emanating from the coil ends along its Z axis. Increasing coil thickness to improve coil performance by tightening up the field produces marginal improvement. But the major disadvantage is that the magnets must be moved further away in order to accommodate the thicker coil. This is not a satisfactory tradeoff since the magnetic field provided by the magnets decreases as the square of the distance from the metallic structure.
An additional difficulty posed by the prior art EMAT structure is that in order to achieve a minimal level of direct coupled signal from the transmitter coil to the receiver coil, the coil turns count must be reduced to enable the coils to be spaced further apart, thus decreasing the defect size resolution. The path length over which the resonance sound field traverses is longer and therefore increase the material's volume under resonance.
In the field of ultrasonics impedance matching is commonly used on a variety of transducers primarily to improve signal to noise performance or to maximize transmitted power. Similarly, in the case of the resonance mode electromagnetic acoustic transducer the ability to selectively tune the impedance of the coil subassembly through a combination of coil construction and capacitative tuning dramatically improves the transfer impedance characteristics between the coil assemblies and the sample under test. This tuning process allows for optimization of signal to noise performance on a variety of materials with differing permeabilities and geometries such as hexagonal shapes.
This tuning process while not essential in all applications adds a margin of safety to the unique end of the sample detection method. Since the RM-EMAT method of generating and detecting sound waves within the test specimen allows for material's property measurement right up to the end of the sample a method to detect when there is sufficient material within the transducer to support resonance was required. Typically external proximity sensors are placed adjacent to the transducer to suppress the measurement system from processing data as the sample enters and exits the transducer, this eliminates false positives from being generated and the needless rejection of good material due to these end effects. However this also creates gaps in the measurement capability. A method has been developed to enable the transducer to “self-sense” the presence of material under test by monitoring the changing coil voltages caused by the changing coil impedances. The means to detect the end of the sample is described. Additionally, since the means and methods using proximity sensors require additional complexity and expense used with the present inventions, the ability of the transducer to self-detect whether or not material is present and properly coupled in the transducer is also of commercial value.